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Novel 2D quantum device concepts enabled by sub-nanometre precision nanofabrication

Periodic Reporting for period 2 - NanoFab2D (Novel 2D quantum device concepts enabled by sub-nanometre precision nanofabrication)

Reporting period: 2018-01-01 to 2019-06-30

Nanotechnology reached a point where we are able to study and modify the structure of materials with close to atomic precision. This opens up entirely new possibilities. We develop novel nanofabrication techniques that are able to modify the structure of materials at or even below the nanometer scale. We apply these methods to a recently discovered new class of materials: two-dimensional (2D) crystals, including but not limited to graphene. The central question of our research is: how can we impart novel functionalities to materials through nanometer precision engineering of their structure?
The available materials and technologies determine our potential for further improving existing applications as well as enabling novel applications. New or improved functionality can result in electronic circuits with ultra-high operation speed and ultra-low power consumption increasing our data processing capability while decreasing the energy consumption, which constitutes a growing problem. Moreover, nanometer scale modification of 2D materials can bring substantial improvement in various applications other than electronics, from advanced sensors to more efficient catalysts.
The overall objective of the project is to develop novel device concept, demonstrate new or improved functionality of graphene and other 2D materials through the nanoscale and atomic level engineering of their structure.
We have designed a new Scanning Tunneling Microscope (STM) setup with a custom designed state-of-the-art optical access to enable the location of nanoscale structures, as well as the atomic scale imaging and tunneling spectroscopy of graphene and other 2D materials and their nanostructures with the capability of spin-polarized measurements.

We have successfully demonstrated the finding back of graphene nanoribbons of less than 10 nm width in our low temperature STM setup. The nanoribbons have to be transferred from another ambient STM setup because the requirements for their lithography and spin polarized measurements are incompatible. This was one of the major technical challenges to enable the spin polarized STM investigations of zigzag graphene nanoribbons fabricated by STM lithography. We could achieve this important technical milestone by developing a gold substrate with a specially designed marker system and exploiting the state-of-the-art optical access custom-designed in our low temperature STM setup.

We have developed a novel device concept based on zigzag graphene nanoribbons with spin-polarized edges that enables the control of both charge and spin signals using a single back-gate electrode in the simplest three-terminal field effect transistor configuration.

We have shown that devices based on zigzag graphene nanoribbons preserve their capability to control the charge and spin signals even up to room temperature that makes them feasible for real-world applications

Besides STM lithography we have further developed the atomic force microscopy nanolithography that enables the nanometer precision engineering of 2D crystals on insulating substrates, which can be directly integrated into electronic devices. Besides the fabrication of graphene nanostructures by AFM lithography, we have also demonstrated the efficient engineering of nanometer-scale strain patterns into graphene sheets by the same technique.

Besides graphene, we also proposed to investigate other 2D materials, such as MoS2 single layers. We have demonstrated that the atomic level modification of 2D MoS2 crystals through the substitution of single S atoms by oxygen can give rise to novel material properties. We have shown that such a peculiar oxygen substitution reaction can spontaneously occur under ambient conditions, and the resulting oxygen substitution sites act as single atomic catalytic centers substantially improving the catalytic activity of MoS2 single layers for the hydrogen evolution reaction.

We have demonstrated the efficient strain engineering of the bandgap of 2D MoS2 single layers by investigating nanometer scale MoS2 bubbles. We could provide unambiguous evidence for the occurrence of the direct to indirect bandgap transition in MoS2 single layers upon 2% biaxial tensile strain, by measuring a smaller electronic bandgap (tunneling spectroscopy) than the optical gap (photoluminescence).
- The Scanning Tunneling Microscope system custom designed for the objectives of this project enables us to fully explore the local electronic structure of 2D materials and their nanostructures and to directly detect the magnetic moments emerging in such nanostructures and their atomic scale defects.

- Our novel device concepts developed based on zigzag graphene nanoribbons with edge magnetism goes beyond the state of the art by enabling an efficient control of both charge and spin signal in a device configuration without the need of external magnetic fields or complex side gate electrode systems. This makes much easier its realization as well as subsequent integration.
- Besides the STM lithography technique we have further developed the Atomic Force Microscope nanolithography for the nanoengineering of 2D materials on insulating substrates directly suitable for integration into electronic devices. We have demonstrated the most efficient way for engineering preferential nanoscale strain patterns into the structure of graphene with controlled symmetry and crystallographic orientation. We have also shown by Raman spectroscopy that such strained graphene corrugations can efficiently scatter charge carriers that can enable their edge free confinement.

- We have also demonstrated that the atomic level modification of 2D MoS2 crystals through the substitution of single S atoms by oxygen can give rise to novel material properties. We have shown that such a peculiar oxygen substitution reaction can spontaneously proceed under ambient conditions on months-long time scale. Most importantly we found that the resulting oxygen substitution sites act as single-atomic catalytic centers substantially improving the catalytic activity of MoS2 single layers for the hydrogen evolution reaction. This is an eloquent example how the single-atom level modification of materials enables new properties and opens up new routes for applications

- We have demonstrated that by reducing the thickness of the graphene naoplateltes incorporated into ceramic composites below 10 layers, the friction coefficient of the composites can be reduced to nearly its half, their wear resistance increased by about twenty times, while their fracture toughness is also .significantly increased.

The results expected in the following are:

- To reveal the electronic and magnetic properties of graphene nanostrcutures and other 2D materials by low temperature and spin polarized STM measurements and spin polarized scanning electron microscopy (SEMPA) measurements.

- Single atom modification of transition metal dichalcogenide single layers and the investigation of the emerging new properties and application potential.

- Realization and demonstration of high quality (beyond state of the art) graphene quantum point contact devices by STM/AFM lithography.

- Inducing heavy strain in the structure of graphene and other 2D materials and investigating its effect on the electronic structure and demonstrating potential applications.
Novel device concept based on zigzag gaphene nanoribbons with magnetic edges.
MoS2-xOx a novel 2D material developed by single-atom level modification of MoS2
The Scanning tunneling microscope equipment specially designed for the objectives of the project.